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Code No: W0221/R07 Set No. 1

II B.Tech I Semester Supplementary Examinations, May 2010ELECTRICAL MACHINES-I

(Electrical & Electronic Engineering)Time: 3 hours Max Marks: 80

Answer any FIVE QuestionsAll Questions carry equal marks

1. (a) Obtain the expression for the mechanical energy developed in electromechan-ical energy conversion devices (Single excited system).

(b) In a electromechanical relay excited from a voltage source, the current andflux linkages are related as i = (1.2)2+3.8(10.2x2); x < 1, find the force onthe armature as a function of . [8+8]

2. (a) What is an equalizer connection? What is necessity of equalizer connection?

(b) An 8 pole DC generator has per pole flux of 40mWb and winding is connectedin lap with 960 conductors. Calculate the generated EMF on open circuitwhen it runs at 400 rpm. If the armature is wave wound at what speed mustthe machine be driven to generate the same voltage. [8+8]

3. (a) What are the causes of sparking in a d. c. machine? Explain how commutationis improved by use of inter poles.

(b) A 22.38 kW, 400-V, 2-pole wave-wound d.c., shunt motor has 840 armatureconductors and 140 commutator segments. Its full-load efficiency is 88% and

the shunt field current is 1.57A. If brushes are shifted backward through 1.5segments from the geometrical neutral axis, find the demagnetizing and dis-torting amp-turn /pole.

[8+8]

4. What is critical speed? How do you draw magnetization characteristics in labora-tory. [16]

5. (a) Explain the exact procedure for connecting a shunt generator in parallel withothers already supplying a load.

(b) Six DC Shunt Generators are running in parallel. Each generator supplies aload current o f 400A at 210V. The shunt field current of each generator is 4A

and the armature resistance of each generator is 0.04 ohm. If one generator issuddenly switched off, determine the % change in terminal voltage, the totalload current being kept unchanged. [8+8]

6. (a) Discuss Armature reaction and commutation in DC motors. Explain theireffects on the performance of the motor and give remedies to their effects.

(b) A 6-pole DC motor has a wave connected armature with 87 slots, each slotcontaining 6 conductors. The flux per pole is 20 m.wb and the armature hasa resistance of 0.13 ohm when the motor is connected to 240V supply and thearmature draws a current of 80A driving a load of 15KW. Calculate

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i. Speed

ii. Armature Torque and

iii. shaft Torque. [10+6]

7. (a) Compare armature voltage control and flux control methods of speed controlof DC motor. Enumerate the advantage and disadvantage of each method.

(b) A 400 V DC shunt motor takes a current of 5.6 A on no-load and 68.3 A onfull load. The load current weakens the field by 3 %. Calculate the ratio offull-load speed to no-load speed, if Ra = 0.18 , brush voltage drop = 2 V, R f= 200 . Also calculate the resistance to be added in series with armature toreduce speed to 50% of no load speed in gross torque remains unchanged.

[6+10]

8. (a) A 220 V, 12 kW shunt motor has a maximum efficiency of 91% and a speed

of 750 rpm., when delivering 85 % of its rated out put. The resistance of itsshunt field is 120 . Determine the efficiency and speed when motor draws acurrent of 75 A from the mains.

(b) A 400 V shunt motor develops an out put of 20 kW when taking 23.25 kW.Field resistance is 210 and the armature resistance is 0.32 . What is theefficiency and power input when, output is 11.5 kW. [8+8]


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1. (a) Explain briefly the principle of the reluctance machine. Give other examplesof reluctance devices.

(b) Draw a schematic diagram indicating flow of energy in the conversion of me-chanical energy to electrical energy. [8+8]

2. (a) What is an equalizer connection? What is necessity of equalizer connection?(b) An 8 pole DC generator has per pole flux of 40mWb and winding is connected

in lap with 960 conductors. Calculate the generated EMF on open circuitwhen it runs at 400 rpm. If the armature is wave wound at what speed mustthe machine be driven to generate the same voltage. [8+8]


(b) A 22.38 kW, 400-V, 2-pole wave-wound d.c., shunt motor has 840 armatureconductors and 140 commutator segments. Its full-load efficiency is 88% andthe shunt field current is 1.57A. If brushes are shifted backward through 1.5

segments from the geometrical neutral axis, find the demagnetizing and dis-torting amp-turn /pole.

[8+8]

4. What is critical speed? How do you calculate the critical speed in laboratory. [16]

5. (a) Discuss in detail the characteristics and applications of DC Shunt, series andcompound generators

(b) Explain the conditions necessary for the parallel operation of DC generators.[10+6]

6. (a) What is the significance of back e.m.f in a DC motor?

(b) Deduce the condition for maximum power of a DC motor.

(c) A 12-pole lap connected 230V shunt motor has 410 conductors. It takes 41A onfull-load. The flux per pole is 0.05 Wb. The armature and field resistances are0.1 ohm and 230 ohms respectively. Contact drop per brush is 1V. Determinethe speed of motor at full-load. [6+6+4]

7. (a) What are the different speed control methods of DC Shunt motor? Explaineach method and enumerate advantages and disadvantages.


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(b) A DC series motor is driving a fan load, whose torque varies a cube of speed.The total armature and series field resistance is 1 . It takes 10 Amp from200 V mains and runs at 1000 RPM. Find the resistance to be connected inseries with the motor to make it run at 800 RPM. [6+10]

8. (a) A 480 V, 20 kW, Shunt motor took 2.5 A when running light. For an armatureresistance to be 0.6 , field resistance of 800 and brush drop of 2 V, findthe full load efficiency.

(b) What is necessity of testing a DC machine? What are the different tests thatcan be conducted on DC machines? [8+8]


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1. With the help of neat diagram obtain the expression for the energy stored in amagnetic system for a simple attracted armature type relay. Explain the operationof system. [16]

2. (a) What is an equalizer connection? What is necessity of equalizer connection?

(b) An 8 pole DC generator has per pole flux of 40mWb and winding is connectedin lap with 960 conductors. Calculate the generated EMF on open circuitwhen it runs at 400 rpm. If the armature is wave wound at what speed mustthe machine be driven to generate the same voltage. [8+8]

3. (a) Explain about demagnetizing Ampere turn per pole and Cross magnetizingAmpere Turn per pole.

(b) What is the purpose of compensating winding? Explain in detail. [8+8]

4. (a) What is critical field resistance? How do you calculate the critical field resis-tance practically.

(b) When a shunt generator fails to build up voltage, then how to rectify the fault.[8+8]

5. Six DC generators are running in parallel, each having an armature resistance of0.15 ohm, running at the same speed and excited to give equal induce e.m.f(s).All generators share load equally at a terminal voltage of 500V. The total load is360KW. If the field current of one generator is raised by 5% and the speed remainsconstant, Calculate

(a) New Terminal Voltage

(b) Output of each machine. [16]

6. (a) What is the significance of back e.m.f in a DC motor?(b) Deduce the condition for maximum power of a DC motor.

(c) A 12-pole lap connected 230V shunt motor has 410 conductors. It takes 41A onfull-load. The flux per pole is 0.05 Wb. The armature and field resistances are0.1 ohm and 230 ohms respectively. Contact drop per brush is 1V. Determinethe speed of motor at full-load. [6+6+4]

7. (a) With neat diagram explain the construction of mechanical starter used forstarting of DC compound motor.


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(b) A 4-pole DC series motor has 944 wave-connected conductors. At a certainload the flux per pole is 34.6 mwb and the total mechanical power developedis 4 kW. Calculate the current taken by the motor and the speed at which itwill run, with an applied voltage of 500 V. The total motor resistance is 3 .

[8+8]

8. (a) A 480 V, 20 kW, Shunt motor took 2.5 A when running light. For an armatureresistance to be 0.6 , field resistance of 800 and brush drop of 2 V, findthe full load efficiency.

(b) What is necessity of testing a DC machine? What are the different tests thatcan be conducted on DC machines? [8+8]


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1. (a) What is field energy? What is the relation of mechanical energy and fieldenergy? Relate them in a linear system.

(b) The magnetic flux density on the surface of an iron face is 1.45 T, find theforce density on the iron face. [8+8]

2. (a) What is an equalizer connection? What is necessity of equalizer connection?(b) An 8 pole DC generator has per pole flux of 40mWb and winding is connected

in lap with 960 conductors. Calculate the generated EMF on open circuitwhen it runs at 400 rpm. If the armature is wave wound at what speed mustthe machine be driven to generate the same voltage. [8+8]


(b) A 22.38 kW, 400-V, 2-pole wave-wound d.c., shunt motor has 840 armatureconductors and 140 commutator segments. Its full-load efficiency is 88% andthe shunt field current is 1.57A. If brushes are shifted backward through 1.5

segments from the geometrical neutral axis, find the demagnetizing and dis-torting amp-turn /pole.

[8+8]

4. (a) What is the necessity of residual pole flux in a self excited generator?

(b) Explain the various methods of connection of the field winding to the armaturein a DC generator. [6+10]

5. (a) What are the reasons for the parallel operation of DC generators?

(b) What are the conditions necessary for parallel operation of DC shunt genera-tors?

(c) Explain the conditions under which two shunt generators operating in paralleldo not share the total load in exact proportion to the machine ratings.[4+4+8]

6. (a) Explain the significance of back e.m.f in a DC motor and explain how itcontrols the value of armature current in DC motor?

(b) A 6-pole lap-wound, 240V motor has the following particulars:No. of armatureconductors = 740; Useful flux / pole = 0.035 WbArmature resistance = 0.3ohm. If the total torque developed by the motor is 140 N-m, find the armaturecurrent taken and the speed of operation. [8+8]


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7. (a) With neat diagram explain the construction of mechanical starter used forstarting of DC shunt motor.

(b) A 4 kW DC series motor has four field coils. The motor runs at 900 RPM andtakes 20 Amp from a 230 V DC source, when field coils are in series undernormal operation. Estimate the speed and current taken by the motor, if fieldcoils are reconnected in two parallel groups of two in series. [8+8]

8. (a) The following readings are obtained when performing a brake test on DC shuntmotor. Spring Balances, 08 kgs and 30 kgs. Diameter of drum = 42 cm. Speedof the motor = 1000 rpm. Applied voltage = 220 V. Line current = 50 A.Calculate out put power and efficiency.

(b) A 500 V DC shunt motor takes a current of 5 A on no-load. The resistancesof the armature and field circuit are 0.2 and 300 respectively. Find,

the efficiency when loaded and taking a current of 125 A and the percentagechange of speed. [8+8]


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